And if your Xbee shield is behaving strangely...

If you are combining the CAN-Bus and Xbee shields from SparkFun, it can be very tempting to plonk the EM406 GPS atop the nice flat prototyping area on the Xbee shield. It's a good fit there! But if stuff suddenly and inexplicably stops working, check that the GPS chassis isn't bridging the terminals on the Xbee shield's RESET switch. I felt like a real idiot :-(